Implement warning for unused dependencies.

This will print a diagnostic for crates which are mentioned as `--extern`
arguments on the command line, but are never referenced from the source.

This diagnostic is controlled by `-Wunused-crate-dependencies` or
`#![warn(unused_crate_dependencies)]` and is "allow" by default.

There are cases where certain crates need to be linked in but are not
directly referenced - for example if they are providing symbols for C
linkage. In this case the warning can be suppressed with
`use needed_crate as _;`.

Thanks to @petrochenkov for simplified core.

Resolves issue #57274

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